Several methods for producing images in the air are known. In skywriting, typically one or more airplanes disperse smoke from behind the airplane as it maneuvers. This trail of smoke forms the image as viewed by an observer. Atmospheric conditions will cause the smoke to disperse, limiting the complexity (e.g., size, shape, color) of images that may be formed using this technique. In addition, images formed from skywriting are not visible in the dark. The efficiency of a system is defined as the total time and cost necessary to display an image at a targeted location. Skywriting is relatively inefficient since it requires an airplane to take off, fly to an intended message position, and then create the image.
Cloud painting offers an alternative to skywriting. In cloud painting, typically a high-powered light is used to project an image onto an existing cloud where light reflects off the cloud toward the observer. Since the light source and observer are both on the same side of the imaging surface, we refer to this herein as front-projection. This technique is more efficient than skywriting and increases the complexity of the image that can be displayed. However, cloud painting can only project an image onto an existing cloud and therefore is not functional when the sky is clear. The ability to project an image at any location is defined as the flexibility of the system. An airplane can be used to produce a smoke cloud at an ideal location, thereby increasing flexibility, but at the expense of efficiency.
In addition, the brightness of the image is reduced as the light beam travels through the atmosphere due to scattering effects. Brightness is inversely proportional to the distance the beam traverses from the source to the observer. Therefore, the higher the altitude of the cloud, the dimmer the image will appear. Also, the higher the altitude of the cloud, the larger the projected image needs to be to appear as the same size when projected on a lower altitude cloud. To form a larger image, the light source must be spread over a larger area, resulting in a further reduction of brightness. Available light sources are not bright enough to compete with direct sunlight, and therefore cloud painting is typically not possible during daylight conditions.
An alternative to cloud painting is image projection onto a water screen. In general, a water screen is produced by spraying a controlled stream of water droplets or water vapor into the air to form a thin screen. To maintain the screen, a continuous supply of water is necessary, along with an apparatus to control the size and position of the water droplets. Water screens suffer from similar flexibility and efficiency constraints as cloud painting.